The present invention is related to a multi-band radio (wireless) terminal apparatus capable of performing radio communications over a plurality of communication frequencies, and more specifically, to a technique suitable in a portable telephone apparatus.
As the communication system available in U.S.A., there is such a communication system that a single CDMA (Code Division Multiple Access) communication method is applied to different communication frequency bands (800 MHz band and 1.9 GHz band). In such a commutation system, if a radio terminal apparatus is operable in both the communication frequency bands, then this radio terminal apparatus becomes very convenient for users.
Generally speaking, this sort of radio terminal apparatus employs the superheterodyne system. This superheterodyne system is applied to each of communication frequency bands so as to switch an intermediate frequency (IF).
In a CDMA type radio terminal apparatus designed for U.S.A. and operable in both communication frequency bands of 800 MHz and 1.9 GHz, a first local oscillator is commonly used in a signal transmission system and also in a signal reception system so as to frequency-convert a radio signal between a radio communication frequency and an intermediate frequency. In this case, the reception frequency is selected to be higher than the transmission frequency by 45 MHz in the 800 MHz band, whereas the reception frequency is selected to be higher than the transmission frequency by 80 MHz. As a result, in the signal transmission system, 130.38 MHz can be commonly used in both the 800 MHz band and the 1.9 GHz as the intermediate frequency. However, in the signal reception system, two sets of intermediate frequencies are provided. That is, the intermediate frequency of 85.38 MHz is provided for the 800 MHz band, whereas the intermediate frequency of 210.38 MHz is provided for the 1.9 GHz band.
As a consequence, two sets of the reception-sided (RX) intermediate frequency (IF) band-pass filters for the 800 MHz band and the 1.9 GHz band are required as to RX IF band-pass filters. Thus, these RX IF band-pass filters for the 800 MHz/1.9 GHz bands must be selectively operated. In general, these RX IF band-pass filters are required to have the steep filtering characteristics. To this end, since SAW (Surface Acoustic Wave) filters are employed as the RX IF band-pass filters, the employment of these SAW filters would increase the manufacturing cost of this sort of radio terminal apparatus.